Gene therapy possibilities for Tay-Sachs disease
Tay-Sachs disease is a devastating, neurodegenerative disorder that (in its more common, infantile form) first presents when an infant is between 3 to 6 months of age and is invariably fatal, usually causing death before the age of five (4). Screening has helped to decrease the prevalence of this calamitous, congenital disease within the United States but there is still no cure available to those with Tay-Sachs disease. Gene therapy however offers a beacon of hope to future generations of Tay-Sachs patients and families. Tay-Sachs Disease Tay-Sachs disease is an autosomal recessive, neurological disorder that causes progressive neuron death. This progressive neural necrosis is mostly confined to neurons within the central nervous system (i.e. neurons within the brain or spinal cord). Neuron death occurs due to the presence of 2 mutated HEXA alleles on chromosome 15 and the subsequent shortage of functional beta-hexosaminidase A, a heterodimer enzyme found within lysosomes. As a heterodimer, beta-hexosaminidase A contains 2 protein subunits: the alpha subunit(encoded by the HEXA gene on chromosome 15)and the beta subunit (encoded by the HEXB gene on chromosome 5) (10). To be operative, beta-hexosaminidase A requires alpha and beta subunits but also functional GM2 activator proteins, which serves as organic enzyme co-factors and also as a transport protein for ganglioside GM2 (7). Beta-hexosaminidase A catalyzes the hydrolysis of ganglioside GM2 and as the brain 20 to 500 times as many gangliosides as non-neural tissue, this enzyme is of particular importance in the central nervous system. Gangliosides are believed to participate in intermolecular interactions and are thought to play a vital role in cell to cell recognition, adhesion and communication (11). When Beta-hexosaminidase A is deficient or defective, an individual's lysosomes are unable to degrade and recycle ganglioside GM2, leading to the toxic build up of ganglioside GM2 within lysosomes. This toxic accumulation of gangliosides within the lysosomes causes the neuron to become distended and eventually causes the neuron to undergo necrosis due to nutritional deficiencies as the neuron (due to its lack of functional lysosomes) is no longer able to catabolize macromolecules for vital nutrients, vitamins and minerals or recycle old and dysfunctional organelles within the cell (9,11). As a result of necrosis, the toxic components within the neuron are released, exasperating the problems of adjacent neurons already experiencing similar difficulties. History Tay-Sachs disease, its progression and differential diagnostic criteria was first described in the early 1880s by two separate physicians for which the condition is named: Warren Tay, an ophthalmologist in Great Britain and Bernard Sachs, an American neurologist practicing in New York. Tay's earliest publications in Volume 1 of the Ophthalmological Society focused primarily on the red "cherry spot" present on the retina of those with Tay-Sachs disease, though he later went on to describe the other clinical symptoms characteristic of the disorder. Sachs's characterized the disorder in a more comprehensive fashion and also noted its higher prevalence among those of Ashkenazi Jewish ancestry in 1887 (1). Epidemiology Tay-Sachs disease was originally thought to be exclusive to Ashkenazi (eastern and central European)Jewish populations when it was first classified the late 19th century, though it is now recognized to be present within all ethnic groups. Tay-Sachs disease is known to be more prevalent among those with Ashkenazi Jewish or French-Canadian heritage and within the Old Order Amish community in Pennsylvania, and the Cajun population of Louisiana. Within these populations, approximately 1 person in 27 carries a defective HEXA allele, meaning that they are a carrier of Tay-Sachs disease. Carrier incidence is also slightly elevated among those with Irish or British Isle decent, with 1 in every 50 persons thought to be a Tay-Sachs carrier (1). In the United States, the incidence of Tay-Sachs carriers is significantly lower as only 1 in 250 individuals is likely to possess a mutated HEXA gene (9). Clinical Features There are 3 different types of Tay-Sachs disease that classd phenotypically by the age of onset, cellular characteristics and progression of the disorder. The most common type of Tay-Sachs disease is infantile Tay-Sachs disease. Infantile Tay-Sachs disease is also the most severe form of the disease, usually resulting in death before the age of 5. Children with infantile Tay-Sachs disease will have inherited 2 noxious HEXA alleles, 1 from each parent. These deleterious alleles are so abnormal that they prevent the formation of any functional beta-hexosaminidase A heterodimers (9) and thus the rapid accumulation of ganglioside GM2 within the neural lysosomes. Children with infantile Tay-Sachs disease will typically develop normally (even though neural death will have begun while the infant was still in the womb)until they reach about 4 months of age. Around this time, the child's mental and physical development will begin to slow and the child's motor muscles will begin to weaken and atrophy. As the disease progresses, the child will gradually begin to regress developmentally, eventually losing their ability to turn over, sit up, crawl or even reach out to loved ones. Children with infantile Tay-Sachs disease will also begin to experience a loss of coordination, a progressive inability to swallow and difficulty breathing. The child's mental and cognitive abilities will deteriorate rapidly and as the disease continues to advance, the infant will begin to develop vision impairment, hearing loss, and paralysis (5,6). By age 2, most children with infantile Tay-Sachs disease will be experiencing recurrent seizures. Eventually, the child will have lost all voluntary muscle function, mental function and their ability to see or hear. By this stage, the child is no longer able to interact with its environment and death is imminent (5). The other 2 types of Tay-Sachs disease, juvenile and adult/late onset, are much less common than infantile Tay-Sachs disease. In these forms of disease, the child has inherited 2 mildly mutated HEXA alleles that allow for a limited production of functional beta-hexosaminidase A. This slows the pernicious accumulation of ganglioside GM2 and thus delays the onset of symptoms(9,2). In juvenile Tay-Sachs disease, symptoms can present anywhere from 2 to 10 years of age while in Late Onset Tay-Sachs disease, symptoms may not become apparent until the individual reaches adolescence or even their third decade of life. Some of the initial symptoms displayed by one with juvenile Tay-Sachs disease include clumsiness or a lack of coordination, muscle weakness, slurred speech and swallowing difficulties. Eventually the child will regress to a stage where they will no longer able to communicate, walk or feed themselves. These children are essentially susceptible to respiratory infections due to aspiration and often will experience recurrent episodes of pneumonia. Children will juvenile Tay-Sachs disease are also likely to experience seizures and most will die before ever reaching adolescence (2, 5). Those with Late-Onset Tay-Sachs disease experience a much milder form of disease. Non-specific symptoms such as muscle cramps and weakness, typically in the legs, often are accompanied by psychological disturbances such as bi-polar disorder and/or psychotic episodes. It is typically very difficult to diagnose those with Late-Onset Tay-Sachs disease as the classical cherry-red spot present on the retina of all infants and most children with Tay-Sachs disease is absent; a definitive diagnosis can be made using a blood test to check the level of Hexosaminidase A in a patients serum, but few physicians will think to administer this blood test as the disorder is so rare. An adult will Late-Onset Tay-Sachs disease will require more mobility assistance (i.e. a wheelchair or a walker) and many will develop speech and swallowing difficulties, though some may live to a normal life expectancy (5). Tay-Sachs Disease and gene therapy In the last 2 decades or so, researches have begun to look hopefully at gene therapy as a means of treating Tay-Sachs disease. The main organization involved in this research is the Tay-Sachs Gene Therapy Consortium. This assembly is affiliated with researchers at a myriad of prestigious institutions, including Auburn University, Boston College, Cambridge University, NYU and Massachusetts General Hospital/Harvard Medical School. So far, researchers within the Tay-Sachs Gene Therapy Consortium have focused their attentions of gene therapy techniques using AAV (Adeno-associated virus) vectors. AAV vectors are (in this instance) Adeno-associated viruses that have been modified to include a functional HEXA haplotype and a promoter (as Adeno-associated viruses naturally have a single-stranded, DNA genome). This AAV vector is injected directly into the brain of the organism affected with Tay-Sachs or some other GM2 Gangliosidosis where it will infect cells indiscriminately, injecting its genome into the cytoplasm's of susceptible cells. Once in the cytoplasm of the infected cell, the genome of the AAV vector will typically remain separate from its host's genome. After being converted to double-stranded DNA by the cell's own machinery, the viral genome can be expressed and functional beta-hexosaminidase A alpha subunits and thus operative beta-hexosaminidase A enzymes can be generated (3). This may seem like a simple enough concept, but unfortunately its medical application is infinity more complex. Proposed Procedures Several hurdles stand between researchers and a viable Tay-Sachs gene therapy technique. The first hurdle to be overcome by researchers was the question of how to administer this supplemental gene into the neurons of the central nervous system, which is protected by a selectively permeable blood-brain barrier. Researchers overcame this barrier by injecting AAV vectors directly into the brains of affected organisms. Researchers then have to contemplate where and how many of these injections should be made. Several researchers at Massachusetts General Hospital, including Dr. Miguel Sena-Esteves, have suggested the thalamus as the primary injection site as the thalamus has a plethora of subcortical and cortical connections (3). The thalamus is the most superior component of the brain stem directly and can be found directly inferior to the third cerebral ventricle, meaning that this neural structure is also ideally located for the secretion-recapture cellular pathway that is the aim of Tay-Sachs gene therapy (in this type of pathway, functional beta-hexosaminidase A enzymes are secreted by cells that have been transformed by the viral vectors and taken up by adjacent, competent cells so that these cells will now possess functional beta-hexosaminidase A enzymes as well) (7). Studies in model organisms have revealed that a single AAV injection can result in diffuse in vivo beta-hexosaminidase A enzyme synthesis and the secretion of functional beta-hexosaminidase A enzymes. These secreted enzymes are distributed to distant neurons via axonal, perivascular (i.e. hematogenous), and cerebrospinal fluid (CSF) pathways. The recapture of these enzymes by such neurons results in the correction of the beta-hexosaminidase A deficiency that causes the deadly disorder. A study preformed by Dr. Cachón-González and associates in 2012 revealed "long-term protein expression by transduced brain parenchyma (8)" and the presence of operative beta-hexosaminidase in choroid plexus epithelium, and dorsal root of ganglia neurons after multiple intracranial injections of AAV vectors containing HEXA and HEXB genes in Sandhoff mice(8). More impressive though is the fact that these mice, who had previously all died within 6 weeks, were able to survive for an unprecedented 2 years after receiving a series of rAAV intracranial injections. Dr. Cachón-González and her fellow researchers at Cambridge University believe that in a human model, rAAV vectors should be injected into the sub-arachnoid space and directly into the cranial parenchymal using a convection-enhanced delivery method if they are to be maximally effective. There has been remarkable, nearly ubiquitous success in mouse models, but unfortunately this success has not translated into universal success in higher mammalian models. Complications While trials in mice and other such model organisms have been exceptionally successful, the same success has not been experienced in non-human primate gene therapy experiments. Just last fall as the Tay-Sachs Gene Therapy (TSGT) Consortium was planning to submit an investigational new drug (IND) application to the FDA for processing, researchers at TSGT ran a final feasibility and safety study in non-human primates only to find that the injection caused severe neurological damage. The thalami of several of the animals to receive the injection had experienced severe neuron loss, leading to profound neurological symptoms. Researchers on the TSGT team speculated that the neuron toxicity observed was the result of massive over expression of beta-hexosaminidase A, which eventually lead to neuron death The research team at TSGT have thus put all clinical trial plans on hold as they attempt to fully understand this adverse neurological response and all modifications that can be done to prevent its re-occurrence in future trials (5). These difficulties may be due in part to the added complexity of higher mammalian brains, though they may also be due to problems with the rAAV vectors themselves (1). Research is currently being conducted by numerous institutions internationally and the hope is to have Tay-Sachs gene therapy clinical trials underway in the near future. References 1. The Cure Tay-Sachs Foundation-Gene Therapy 2. The Cure Tay-Sachs Foundation- History 3. First Human Gene Therapy Trial Planned For Deadly Tay-Sachs Disease 4. PubMed Health: Tay-Sachs Disease 5. Tay-Sachs Gene Therapy Consortium 6. Genetics Home Reference 7. Effective gene therapy in an authentic model of Tay-Sachs-related diseases 8. Gene transfer corrects acute GM2 gangliosidosis-potential therapeutic contribution of perivascular enzyme flow. 9. DNA Learning Center 10. GeneCards 11. Gangliosides